This invention relates to molded thermoplastic foam.
More specifically, this invention pertains to molded thermoplastic foam which exhibits enhanced flame retardant properties.
The manufacture of molded articles from expanded thermoplastic particles is well known. The most commonly used thermoplastic particles are expandable polystyrene beads known as EPS. Typically, polystyrene beads are impregnated with a blowing agent which boils below the softening point of the polystyrene and causes the impregnated beads to expand when they are heated. When the impregnated beads are heated in a mold cavity, they expand to fill the cavity and fuse together to form a shaped article.
The formation of molded articles from impregnated polystyrene beads, is generally done in two steps. First, the impregnated polystyrene beads are pre-expanded to a density of from about 1 to about 3 pounds per cubic foot. Second, the pre-expanded beads are heated in a closed mold to further expand the pre-expanded beads and to form a fused article having the shape of the mold. The second step is generally referred to as molding.
The pre-expansion step is conventionally carried out by heating the impregnated beads using any conventional heating medium such as steam, hot air, hot water, or radiant heat. One generally accepted method for accomplishing the pre-expansion of impregnated thermoplastic particles is taught in U.S. Pat. No. 3,023,175 to Rodman.
There are two commonly used methods of incorporating flame retardant agents for use in expandable vinyl aromatic monomer polymers. The first method is the one step process where the flame retardant is added to the vinyl aromatic monomer during the suspension polymerization followed by the addition of the blowing agent. The second method is a two step process where the vinyl aromatic monomer is suspension polymerized to form hard polymer beads which are screened to the desired particle size. In the second step the beads are resuspended and the flame retardant and a blowing agent are added and incorporated during this impregnation.